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A naturally aspirated in-line six-cylinder 2.9-litre Volvo engine is operated in Homogeneous Charge Compression Ignition (HCCI) mode, using camshafts with low lift and short duration generating negative valve overlap. This implementation requires only minor modifications of the standard SI engine and allows SI operation outside the operating range of HCCI. Standard port fuel injection is used and pistons and cylinder head are unchanged from the automotive application. A heat exchanger is utilized to heat or cool the intake air, not as a means of combustion control but in order to simulate realistic variations in ambient temperature. The combustion is monitored in real time using cylinder pressure sensors. HCCI through negative valve overlap is recognized as one of the possible implementation strategies of HCCI closest to production. However, for a practical application the intake temperature will vary both geographically and from time to time.

The combustion processes of three injection strategies in a light-duty (LD) diesel engine at a medium load point are captured with a high speed video camera. A double-pilot/main/single-post injection strategy representative of a LD Euro 6 calibration is considered as the reference. There is a modest temporal spacing (dwell) after the first pilot (P1) and second pilot (P2). A second strategy, “A,” adds a third pilot (P3). The dwell after both P2 and P3 are several times shorter than in the reference strategy. A third strategy, “B,” further reduces all dwells. Each injection has its own associated local peak in the heat release rate (HRR) following some ignition delay. Between these peaks lie local minima, or dips. In all three cases, the fuel from P1 combusts as a propagating premixed flame. For all strategies, the ignition of P2 primarily occurs at its interface with the existing combustion regions.

Heat transfer losses are one of the largest loss contributions in a modern internal combustion engine. The aim of this study is to evaluate the contribution of the piston bowl type and swirl ratio to heat losses and performance. A commercial CFD tool is used to carry out simulations of four different piston bowl geometries, at three engine loads with two different swirl ratios at each load point. One of the geometries is used as a reference point, where CFD results are validated with engine test data. All other bowl geometries are scaled to the same compression ratio and make use of the same fuel injection, with a variation in the spray target between cases. The results show that the baseline case, which is of a conventional diesel bowl shape, provides the best emission performance, while a more open, tapered, lip-less combustion bowl is the most thermodynamically efficient.